Police in Israel are on the hunt for two suspected terrorists as an apparent bomb explosion aboard a civilian bus in Tel Aviv injured at least 21 people.

The terror attack was the first of its kind in Tel Aviv since 2006.

According to the latest casualty reports, one of the passengers, a teenager, was seriously injured, while another, also a teenager, sustained moderate injuries. The rest were lightly wounded, and at least six have already been released from the hospital.

The bomb, which exploded around noon local time on bus number 142, is believed to have been hurled onto the bus inside a backpack. A second, unexploded device was reportedly found on the same bus.

A suspect who was arrested shortly after the attack was released, and police say at least one terrorist may be on the loose in the area, and is being considered armed and dangerous.